1. Field of the Invention
The invention is directed generally to a feed circuit in communication systems. More specifically, the circuit is for the remote feed of connected communications terminal devices. A dc voltage source is connectable to a subscriber line or disconnectable from the subscriber line by using an electronic switch via windings of an input transformer connected to the subscriber line. The switching input of the electronic switch is connected to one hole of the dc voltage source, and the switch output thereof is connected to a winding of the transformer. The dc voltage source is switchable via the control input thereof.
2. Description of the Related Art
Such a feed circuit is disclosed in a publication of Siemens AG, "IC's for Communications", 1989, pp. 244-257. The feed circuit indicated in FIG. 1 therein has an integrated circuit PEB 2025 "ISDN exchange power controller (IEPC)". An electronic switch is provided in this integrated circuit for every subscriber line, and the switching input of this electronic switch is connected to a pole of the dc voltage source and the switch output thereof is connected to a winding of the input repeater connected to the subscriber line. The dc voltage source is connectable via the control input thereof with a control logic.
In case of an overload, for example, given a short on the subscriber line, the overload procedure described on page 249 of the above-referenced publication sequences. In this overload procedure, high dissipated power occur, particularly in the first 200 ms. The electronic switch is capably-sized such that the high dissipated powers do not lead to the destruction of the switch. This means that given a short, the dissipated power defines the size of the electronic switch. Consequently, high-cost electronic switches, which have properties that lie far beyond the properties necessary during normal operation, are required.